Nuclear isomers are long-lived nuclear excited states that have the same atomic and mass numbers as those of the ground state. Isomers were discovered in 1921 and are currently believed to exist as a consequence of a low excitation energy and a high spin quantum number, I. This combination of low excitation energy and high spin makes the decay of nuclear isomers by .gamma.-ray emission or by conversion-electron emission much slower than for conventional excited states. Because high spin is partly responsible for their long lifetime, such nuclear isomers are frequently called spin isomers (or K-isomers). A nuclear isomer having a lifetime of 44 years, such as .sup.178m Hf, combines a high spin (I=16) with a relatively high excitation energy (2.4 MeV), which is still much lower than the neutron separation energy (S.sub.n =7.6 MeV) for this species.
Another type of nuclear isomer, .sup.242m Am, was found in 1962, in attempts to synthesize very heavy elements. See, "Nuclear Fission," A. Michaudon, in Advances in Nuclear Physics, M. Baranger and E. Vogt, Eds. (Plenum Press, 1973), Vol. 6, p. 1. This isomer does not have high spin, has a relatively high excitation energy (2.9 MeV), and was observed to decay by fission, as opposed to .gamma.-ray emission. Since this species is not a spin isomer, these observations were subsequently interpreted to be properties of highly deformed matter. Potential energy surfaces (PES) for actinide nuclei are known to possess a second well for large deformations in addition to a first well at ground-state deformation (see, e.g., Michaudon, supra). The .sup.242m Am isomer is interpreted as being in a superdeformed (SD) nuclear state when characterized by this second well and, for this reason, is called a shape isomer. The inner barrier in the PES between the first and second wells retards .gamma.-ray decay of shape isomers which, therefore, preferentially decay by fission through the outer barrier. About 25 shape isomers, also called fission isomers, because they decay principally by fission, have been discovered to date for actinide nuclei generally grouped between uranium and curium in the Periodic Table.
Since 1986, many SD rotational bands have been observed for nuclei having masses between A.apprxeq.150 and A.apprxeq.190 (generated using heavy-ion-induced reactions). The results of PES calculations strongly suggest the existence of a second well in the PES for these nuclei. See, e.g., "Superdeformed Nuclei," Robert V. F. Janssens and Teng Lek Khoo, Annu. Rev. Nucl. Part. Sci. 41, 321 (1991). Therefore, shape isomers, similar to fission isomers but with smaller atomic numbers, are likely to exist in the A.apprxeq.150 and A.apprxeq.190 mass regions, but their decay by fission is inhibited by their outer fission barrier, which is much higher than for actinide nuclei. Although less likely, shape isomers may also exist in other mass regions. Most postulated shape isomers are expected to have an excitation energy E.sub.exc smaller than S.sub.n and would decay by .gamma.-ray emission in a similar manner to spin isomers.
Some shape isomers may have an energy E.sub.exc greater than S.sub.n, however. This property, which makes the decay of these isomers by spontaneous neutron emission possible, is supported by PES calculations for nuclei in the A=200 region. For example, some mercury isotopes have shown deep second wells with E.sub.exc of the order of 10 MeV. Other nuclei may present similar properties. See, e.g., "Super-Deformation and Shape Isomerism: Mapping the Isthmus," by S. J. Krieger et al., Nucl. Phys. A542, 43 (1992) and "Isomeres De Forme Dans Les Noyaux Pairs-Pairs: Premiere Selection De Candidats Dans La Region De Masses A&lt;208," by Michel Girod et al., Centre d'Etudes de Bruyeres-le-Chatel Note No. CEA-N-2560 (May, 1998). The neutron decay of these shape isomers should make it possible for them to be useful for both neutron and energy sources. Shape isomers having E.sub.exc &lt;S.sub.n may also be of interest. For convenience, (N,Z).sub.n,is shape isomers that have neutron number N, proton number Z, and E.sub.exc &gt;S.sub.n (E.sub.exc &lt;S.sub.n) are called N-isomers (n-isomers) in what follows.
An incident neutron interacting with an isomeric state may be inelastically scattered with an outgoing energy greater than the incident energy because the initial isomeric state of the nucleus can make a transition to a lower-energy state during the interaction. This type of neutron acceleration (called superinelastic scattering), which cannot occur for target nuclei in their ground state, is however predicted by theory and has been experimentally verified for a few spin isomers. For these isomers, neutron acceleration is limited by the small angular momentum carried by the incident neutron, which can therefore cause only low-energy transitions from the isomeric state to other excited states having lower energy, but high spin. In most isomers, states reached in the residual nucleus after neutron acceleration also have a relatively high excitation energy and decay by prompt .gamma.-ray emission, thus liberating most of the energy initially stored in the spin isomer. Superinelastic scattering is also possible with N-isomers with possible greater neutron acceleration than with K-isomers because transitions of the N-isomer to lower-energy states are not limited by the same spin and energy considerations. In addition, the high excitation energy of the N-isomer makes the reaction (n,2n) and neutron multiplication possible, even for incident neutrons with low energies. The exact properties of these reactions depend on the intrinsic properties of the N-isomers (excitation energy and shape of the PES) and on the incident energy of the neutron.
N-isomers have lifetime, yield, and neutron-energy spectrum properties that could make them useful as neutron sources. N-isomers might also be used as neutron multipliers [through the use of (n,2n) reactions] and as "neutron accelerators" (through superinelastic scattering). Such neutron sources, depending on their specific properties and on their availability, could supplement existing neutron sources which rely on radioactive substances mixed with materials with a low neutron-emission threshold (like beryllium), or on fission (like .sup.252 Cf). As an example, a source containing 1 g of N-isomers having A.apprxeq.190 and a lifetime of 1 yr. would emit neutrons at a rate of about 10.sup.14 n/s and low-energy .gamma.-rays at a similar rate. By comparison, a .sup.252 Cf fission source emits at most about 10.sup.10 n/s (for a quantity of 5 mg), which is 4 orders of magnitude below the above intensity quoted for N-isomers. Large quantities of .sup.252 Cf are unavailable because these nuclei are generated from a long neutron-irradiation chain, which involves a sequence of ten neutron captures with four intervening .beta.-decays after the process is started with the irradiation of .sup.242 Pu in the high neutron flux of a fission reactor. It is anticipated that the formation of N-isomers would be simpler than for the formation of .sup.252 Cf and that larger quantities of N-isomers are possible to produce than can presently be obtained for .sup.252 Cf. Radioactive neutron sources based on (a,n) reactions induced by .alpha.-ray emitters can produce up to about 10.sup.8 n/s and are therefore less intense by about 2 orders of magnitude than .sup.252 Cf sources. The energy of the neutrons emitted by N-isomers is difficult to predict, because it partly depends on the energy difference E.sub.exc -S.sub.n. Therefore, neutron sources based on N-isomers have potential as intense neutron sources beyond that which is possible from existing neutron sources. Because of the nature of the phenomenon of neutron acceleration, N-isomers and n-isomers could also be used to harden the energy-spectrum of neutron sources more effectively than K-isomers.
Neutrons emitted by N-isomers could release energy through loss of kinetic energy by nuclear collisions in the source. But additional energy could also be liberated as a result of reactions induced by the emitted neutrons in the source itself or in materials included in the source. Such energy sources might be bulky because of the long mean-free-path of neutrons in matter. Energy may be released from .gamma.-rays emitted by neutron radiative capture or neutron inelastic scattering in the source materials. The magnitude of the capture cross sections and the total energy and multiplicity of the capture .gamma.-rays would be important criteria in selecting suitable materials. Radiative-capture rates could be increased by decreasing neutron energy using hydrogenous compounds in the source as neutron moderators. As an illustration, an N-isomer neutron source emitting 10.sup.14 n/s would generate about 100 W per g of N-isomers from capture .gamma.-rays, assuming that all emitted neutrons would be absorbed by radiative capture. By comparison, a .sup.252 Cf source produces 39 W/g from spontaneous fission (3 times less) but, as noted above, this source cannot be produced in large quantities. The .alpha.-radioactivity of .sup.238 Pu is currently used as an energy source but produces 0.6 W/g, a factor of about 65 below that of .sup.252 Cf. Larger energy release from N-isomers is also possible by using the emitted neutrons from the source to induce fission in a fissionable material added to the source, but with the disadvantage of producing fission products, as for .sup.252 Cf .
Inclusion of K-isomers in the source might also generate additional energy by liberating the huge stored energy in these isomers, using reactions induced by neutrons emitted by the N-isomers. As an illustration, an energy of 1.3.times.10.sup.3 MJ would be stored in 1 g of .sup.178m Hf. These neutrons could cause the K-isomers to make transition to excited states which would subsequently decay to the ground state by .gamma.-ray emission, thereby releasing energy, while one neutron would be re-emitted in this interaction. This same neutron could then be used to release energy through other similar interactions or radiative capture.
Accordingly, it is an object of the present invention to use N-isomers as a source of spontaneous neutrons and as a source of energy.
Yet another object of the invention is to use neutrons emitted from N-isomers, or from another neutron source, to liberate the energy stored in K-isomers to supplement energy sources derived from radiative capture, fission, or other reactions in materials included in the source.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.